PHYSICO-CHEMICAL CONDITIONS STUDY FOR DEPOSITION OF SILICON LAYER ON A SUBSTRATE BY RF PLASMA

The RF plasma process is used to purify and to deposit silicon powders on a substrate. The experimental conditions used (Argon plasma, P =25 kW, d(injection) =20 cm) lead to a high deposition rate (600 µm.h⁻¹) of dense polycrystalline silicon. The nature of the substrate seems to be of great importance for the deposit quality: in the case of an alumina substrate, the surface of the deposit has a high porosity but for a mullite substrate, the porosity disappears. The efficiency of the mass deposition is close to 50 % for the injection of particles with an average diameter close to 100 µm, for a mass flow rate of 0.1 g.mn⁻¹ on a 2×2 cm² substrate.
The plasma process allows a local injection of silicon powders in the plasma which leads to the purification of the material. The LDA measurements in the plasma flow permitted the determination of the trajectories and the velocity (24 m.s⁻¹ for a residence time (τ) of 10 ms in the jet) of the injected silicon particles. The displacement of the substrate under the plasma flow leads to a high purification of the deposit by a mechanism close to a zone refining (SEM/EDX analysis).
The introduction of Hydrogen in the plasma during the treatment improves the photovoltaic properties of the deposited silicon. The identification of the different hydrogen excited species of the plasma has been undertaken by OES.